Many processes exist for drying products, such as various foods, fruits, vegetables, and other biological substances. For example WO89/08229 discloses a system and a method for drying granular material, in which the granular material is subjected to a drying agent, such as nitrogen.
It is also known to freeze-dry vegetables. Such a method is known to cause rupture of the cells and to increase the permeability of vegetables to water, see U.S. Pat. No. 4,788,072 at Column 2, Lines 15 to 20.
It is further known to dry fruit or vegetable slices by immersing said slices into a sugar solution, see for example, European Patent Specification No. EP-A-0,339,175. When using a sugar solution with a high Dextrose equivalent (DE) value, for example, a DE value of 70, low molecular weight sugars are able to penetrate into the cells, whereby a higher sugar content exists. Such dried products may not have a water activity lower than 0.4, and some of the contents of the cell, such as flavour and odour constituents, may no longer be present in the interior of the cell.
When using a sugar solution with a low DE value, for example, a DE value of 25, sugars are extracted from the product. This dried product also may not have a water activity lower than 0.4.
Furthermore, it is considered that due to the extraction of sugars, the taste and odour characteristics of the product may be altered.
The sugar osmotic drying is also considered as a pre-treatment, that is, a treatment for lowering the water content of the product before a final drying treatment, such as frying. The water content of the osmotically dried product may thus not be low enough to preclude spoilage of the product without additional measures, such as refrigeration, sterile packaging or the addition of preservatives. Due to the treatment of the product in a liquid sugar solution, sugar may completely fill the voids or spaces situated between two adjacent cell membranes.
It is further known to dry grapes by spraying them before harvesting with a composition to facilitate removal of water from the grapes, see U.S. Pat. No. 5,068,988. The dried grapes retain a sufficiently high water content so as to produce sufficient juice for making wine. The wines obtained by using such dried grapes have a high alcohol content.
It is also known to dry timber using a carefully regulated temperature and humidity regime during the drying process. The temperature and humidity regime is species- and size-specific and is selected to ensure that the drying operation does not cause warping of the product as a result of excessive moisture and temperature gradients within the material. The drying process results in a longer drying time than would result from drying without humidity regulation. Details of such drying processes are presented in a large number of standard works, for example, in the 1991 ASHRAE Handbook, HVAC Applications.
It is generally accepted in the known art that "Based on the analysis of heat and mass transfer, the most efficient dehydration systems will maintain the maximum vapour-pressure gradient and the maximum temperature gradient between the air and the interior parts of the product", see Introduction to Food Engineering, R. P. Singh and D. R. Heldman, Academic Press (1993), at Page 422.
It is also generally accepted that in known drying processes the outer layer of the product becomes essentially impermeable to aroma compounds but still transmits some water vapour to allow drying to continue, see for example, "Food Dehydration", G. V. Barbosa-Canovas and M. R. Oaks eds., A.I.Ch.E. Symposium Vol 89 (1993), Page 32.
Finally, PCT Application Specification No. WO 94/13146 of one of the co-inventors of the present invention discloses a method and an apparatus for dehydrating biological products in which a closed system is used so as to ensure the retention of the essential flavour and fragrance of the natural product.